Conventionally, in image viewing displays, such as LCD, an indoor lighting such as a fluorescent light, or incidence from sunlight from windows, or a reflection of operator's shadow etc. given to the viewing display surface sometimes aggravates visibility of pictures. Therefore, an light diffusion layer is provided, on display surface, in which minute unevenness structure showing antiglare property, formed in order to diffuse a surface reflected light, to stop a regular reflection of outdoor daylight and to prevent a reflection of outside environment (having antiglare property) for the purpose of improvement in the visibility of images. As a formation method of light diffusion layer, a method is mainly used in which a resin layer is formed from a coated layer having a resin containing fine-particles in dispersed state, because fine structure may be easily realized and at the same time excellent productivity may be realized by the method.
If a light diffusing layer is mounted on LCD, however, surface scattering occurs, which depends on conditions for forming the light diffusing layer, to thereby render a display screen in black display whitish, that is cause so-called white blurring, leading to a problem to reduce contrast in screen display.
For example, disclosed in JP-A No. 10-20103 is a light diffusing layer with improved antiglare property and the like, and having 60° gloss value on a surface thereof to be adjusted 90% or less paying attention to the 60° gloss value, whereas a great amount of particles small in average diameter is used in the light diffusing layer as stated in the published patent application, therefore causing white blurring. Further disclosed in WO 95/31737 is a light diffusing layer having 60° gloss value on a surface thereof to be adjusted 90% or less, whereas the light diffusing layer cannot be said to be sufficiently improved on suppression of white blurring.
As a method improving white blurring, a method has been available in which a low refractive index layer having a refractive index lower than that of a resin coat layer is provided on a surface of the resin coat layer with a minute unevenness structure to thereby impart an antireflection function thereto and to prevent reduction in contrast in screen display. In order to reveal an antireflection function, however, it is necessary to usually provide a uniform low refractive index layer having a thickness ¼ times a wavelength, and if an antireflection treatment is performed in a wet treatment on a minute unevenness structure, a thickness differs at peaks and valleys in minute unevenness structure due to leveling of the low refractive index layer, thereby disabling an antireflection function to be sufficiently revealed.
As a transparent film on which a light diffusing layer is formed, there have been generally employed films made of materials excellent in transparency, such as triacetyl cellulose, polyethylene terephthalate, polycarbonate and the like. Especially, a triacetyl cellulose film low in birefringence is mainstream as a material of a transparent film employed in a polarizing plate indispensable in LCD. In a case where being exposed to a humidified environment at a high temperature for a long time, however, a light diffusing sheet obtained by forming a minute unevenness shape on a surface of a triacetyl cellulose film has had a problem of peeling-off of and cracking in the light diffusing layer due to hydrolysis of the triacetyl cellulose film.
There has been available a transparent film that is not hydrolyzed in a humidified environment at a high temperature. As such a transparent film, there has been generally employed a stretched film having an improved mechanical strength. A stretched film generates a phase difference because of stretching, however. Therefore, in a case where a stretched film is used as a protective film for a polarizer, an inconvenience arises that a view angle characteristic is reduced. While recently, a norbornene-based resin film has been employed as a transparent film hard in revelation of a phase difference even after stretching and excellent in optical characteristic, the norbornene-based resin film on which a resin coat layer is provided for light diffusion is poor in adherence to the resin coat layer, being resulted in difficulty forming a light diffusing layer.
It is an object of the present invention to provide a light diffusing sheet, even in a case where the present invention is applied to a high definition LCD, maintaining antiglare property, showing almost no white blurring to be recognized due to surface scattering, exhibiting almost no birefringence, and being excellent in adherence and durability. It is another object of the present invention to provide an optical element in which the light diffusing sheet is provided and it is still another object of the present invention to provide an image display employing the optical element.